Short answer
A fake IW371642 shows itself in four places before anyone opens the case: the subdials, the dial printing and hands, the case and crystal profile, and the caseback engraving. On a genuine Portugieser Chronograph the subdial at 12 counts chronograph minutes, the subdial at 6 runs the seconds, and the reset snaps back to zero in one motion. On most replicas one of those two subdials is decorative, or the counter at 12 never advances when you start the chrono.
What the IW371642 is
The IW371642 sits in the IWC Portugieser Chronograph family, reference block IW3716: a 41 mm steel case, about 13 mm thick, a lacquered dial with applied numerals, leaf hands, a date at 3, two subdials at 12 and 6, and a box shaped sapphire crystal. The last digits of the reference mark dial and strap variants, so start any comparison from photos of that exact variant, not from a generic Portugieser photo.
Where replicas give themselves away
- Subdial layout. Genuine: chronograph minutes at 12, running seconds at 6. Many fakes turn the 6 o'clock subdial into a 24 hour indicator or a second time zone, a clear tell in a short video.
- Subdial spacing. The gap between the two subdial centers follows the movement inside. Copies built on a different chronograph base sit a fraction off, which shows when you overlay a straight on photo.
- Dial printing. Look for sharp edges on IWC SCHAFFHAUSEN, even stroke weight and no ink pooling in the small letters. Weak printing smears at macro distance.
- Hands and numerals. Leaf hands with crisp bevels, applied numerals with polished facets. Fakes show flat, rounded or over polished edges.
- Crystal. A domed box sapphire with anti reflective coating on both faces. Flat mineral glass, a purple tint or heavy reflections point to a copy.
- Case and pushers. Caseback curvature and side profile matter. Pushers should start the chrono with a smooth push and reset with a single firm click.
- Caseback engraving. Depth, font and spacing of the reference and serial lines. Laser etching looks shallow next to engraved markings.
- Movement finish. Rotor engraving, striped plates, blued screws and clean perlage. A stock industrial chronograph with no decoration is not an IWC calibre.
Inspection procedure
- Ask for a straight on dial photo in diffused light and a second photo at an angle.
- Start the chronograph, run it for 60 seconds, then confirm the counter at 12 has moved one minute.
- Press reset and watch the hand return to zero in one move, not drift or hesitate.
- Watch the subdial at 6 for 30 seconds and confirm it is running seconds only.
- Measure case thickness at the bezel edge with calipers and compare it with the published figure.
- Check the crystal edge for a box dome and look for color tint in the glass.
- Photograph the dial at macro distance and compare the printing, numerals and hand bevels.
- Turn the watch over and inspect the caseback engraving depth, font and reference placement.
- Request a movement photo and check rotor engraving, plate finish and screw color.
- Inspect the strap and buckle markings, then weigh the watch and compare it with a known figure.
Buying a replica IW371642 with open eyes
Ask the seller for a video of the chronograph running and resetting, not stock photos. Get written confirmation that both subdials work as intended, and expect a third party chronograph movement, not an IWC calibre. Confirm the case size, the crystal type and the strap material before you pay, and treat any listing that hides the caseback as a warning sign.